Problem 2.185

A saturated water vapour is contained in a cylindrical vessel under a weightless piston at a temperature $t=100^{\circ} \mathrm{C}$. As a result of a slow introduction of the piston a small fraction of the vapour $\Delta m=0.70 \mathrm{~g}$ gets condensed. What amount of work was performed over the gas? The vapour is assumed to be ideal, the volume of the liquid is to be neglected